Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
Spray nozzle tips for these spraying techniques are generally made having varying orifice sizes and angles to accomodate the coating formulation to be sprayed, the amount to be sprayed, the surface area to be covered, the desired thickness, and the like.
The proper selection of a spray nozzle tip is determined by the fan width for a particular application and by the orifice size which will provide the desired amount of coating formulation and accomplish proper atomization of the material.
Typically, for relatively low viscosity formulations, nozzle tips having smaller orifice sizes are generally preferred. For higher viscosity formulations, nozzle tips having larger orifice sizes are desired. The quantity of fluid sprayed is generally determined by the size of the orifice while the thickness of the applied coating is typically determined by the orifice size and the angle of the spray fan that is produced, conventionally known in the art as the "spray angle". Thus, two nozzle tips having the same orifice size but different spray angles will deposit the same amount of coating formulation but over different surface areas.
For a given set of spraying and coating formulation conditions, it is generally desirable to utilize a nozzle tip which offers as wide a spray as possible so as to coat a larger surface area per unit of time.
The spray angle, which determines the width of the spray measured at a given distance from the nozzle orifice, is a function of the manner in which the orifice is cut into the nozzle tip. Many different airless-type nozzle tip designs are in use today but they generally all require a "V"-type cut to be made in the tip. The depth and angle of the "V"-type cut determines the spray angle and, correspondingly, the width of the spray that is obtained. In general, the deeper and narrower the "V"-type cut, the wider the flat, fan-shaped spray pattern that is produced. However, nozzle tips having large spray angles are difficult and expensive to manufacture. The required deeper and narrower "V"-type cuts are more difficult to make, particularly when made from the typical construction materials that are used for such nozzle tips, for example, tungsten carbide, which is quite brittle.
Thus, airless spray nozzle tips are generally of a type which are capable of producing a flat, fan-shaPed spray pattern having a spray angle which typically is no greater than about 70.degree. to about 80.degree. with orifice sizes ranging from about 0.007 to about 0.072 inches.
While attempts have been made to provide airless nozzle tips which produce wider spray Patterns, most have not met with very much success. Moreover, essentially all of these attempts have focused upon mechanical solutions, namely, providing changes in either the construction of the nozzle guns, the nozzles, and/or the nozzle tips, to attempt to provide such desired wider spray patterns. Even when some success is obtained in producing a wider spray angle, such as discussed in for example U.S. Pat. No. 4,097,000, the costs associated with manufacturing and using such modified nozzle and/or nozzle tip arrangements are generally economically prohibitive recognizing that nozzle tips generally have a relatively short useful working life.
Moreover, regardless of the particular nozzle tip being used, if a different spray width is desired, it is generally necessary to change the nozzle tip to provide such a new spray width. Thus, a typical nozzle tip is rated to provide a particular spray width under proper spraying conditions. It generally cannot substantially vary from producing a spray width other than that which it is rated. A change in the spray width cannot, therefore, be obtained while spraying although such flexibility may be quite desirable in some instances.
Clearly, what is needed is a means for providing even wider airless spray patterns using airless spray techniques. Desirably, such means should also be capable of varying the width of the spray during a spraying operation without the need for having to change the nozzle tip. Most desirably, such means should be capable of accomplishing these objectives using conventional airless spray guns, nozzles and nozzle tips without having to modify them in any manner.